In the text hereafter, what is referred to as CLC (Chemical Looping Combustion) is an oxidation-reduction or redox looping method on an active mass. It can be noted that, in general, the terms oxidation and reduction are used in connection with the respectively oxidized or reduced state of the active mass. The oxidation reactor, also referred to as air reactor, is the reactor where the redox mass is oxidized and the reduction reactor, also referred to as combustion reactor or fuel reactor, is the reactor where the redox mass is reduced.
The CLC method allows to produce energy from hydrocarbon-containing fuels while facilitating capture of the carbon dioxide emitted during combustion.
The CLC method consists in using redox reactions of an active mass, typically a metal oxide, for splitting the combustion reaction into two successive reactions. A first oxidation reaction of the active mass, with air or a gas acting as the oxidizer, allows the active mass to be oxidized. A second reduction reaction of the active mass thus oxidized, by means of a reducing gas, then allows to obtain a reusable active mass and a gas mixture essentially comprising carbon dioxide and water, or even syngas containing hydrogen and carbon monoxide. This technique thus enables to isolate the carbon dioxide or the syngas in a gas mixture practically free of oxygen and nitrogen.
The combustion being globally exothermic, it is possible to produce energy from this process, in form of vapour or electricity for example, by arranging exchange surfaces in the active mass circulation loop or on the gaseous effluents downstream from the combustion or oxidation reactions.
It is also possible to consider producing syngas or even hydrogen with such a CLC method by controlling the combustion and carrying out the required purifications downstream from the combustion process.
U.S. Pat. No. 5,447,024 describes for example a chemical looping combustion method comprising a first reactor for reduction of an active mass by means of a reducing gas and a second oxidation reactor allowing to restore the active mass in its oxidized state through an oxidation reaction with wet air. The circulating fluidized bed technology is used to enable continuous change of the active mass from the oxidized state to the reduced state thereof.
The active mass going alternately from the oxidized form to the reduced form thereof and conversely follows a redox cycle. It acts as an oxygen carrier by yielding its oxygen in a reduction zone (reduction reactor) under suitable operating conditions, and it is subsequently conveyed to an oxidation zone (oxidation reactor) where it is reoxidized on contact with an oxidizing gas (such as air or water vapour for example).
Thus, in the reduction reactor, active mass (MxOy) is first reduced to the state MxOy-2n-m/2 by means of a hydrocarbon CnHm that is correlatively oxidized to CO2 and H2O, according to reaction (1), or optionally to a mixture CO+H2, depending on the proportions used.CnHm+MxOy→nCO2+m/2H2O+MxOy-2n-m/2  (1)
In the oxidation reactor, the active mass is restored to its oxidized state (MxOy) on contact with air according to reaction (2), prior to returning to the first reactor.MxOy-2n-m/2+(n+m/4)O2→MxOy  (2)
In the above equations, M represents a metal.
The reaction zones allowing chemical-looping combustion reactions to be conducted generally consist of fluidized beds or circulating fluidized beds, also referred to as transported beds.
The combustion of solid hydrocarbon feeds such as coal causes CLC plant fouling problems.
One major problem likely to arise upon the combustion of solid hydrocarbon feeds lies in the sticking of the solid fuel injected into the CLC plant, notably at the injector allowing the feed to be sent into the combustion reactor. In the case of gravity injection of coal into a fluidized bed for example, the significant heat generated in the injector, by conduction with the high-temperature bed of the combustion reactor, can lead to sticking of the crushed coal and cause an obstruction in the injector. A direct consequence of such an obstruction is the stoppage of the CLC unit and the implementation of a maintenance operation. This fouling phenomenon due to sticking of the feed is all the more significant as the proportion of volatile matter is large in the feed, as it is for example the case with biomass.
Another problem relates to the formation of ashes in the combustion reaction zone, and more particularly the formation of agglomerated bottom ash whose agglomeration may “foul” the reactor. This phenomenon affects the smooth operation of the CLC process and it may require complete stoppage of the plant for maintenance. Agglomerated bottom ash is a specific feature of the combustion of solid hydrocarbon feeds such as coal. Indeed, the mineral material content of solid fuels is not insignificant and, once combustion of the carbon and of the hydrogen is completed, solid residues called ashes form. Table 1 groups the analyses of two coals A and B by way of example. It can be observed that the ash content of the coals varies depending on the origin of the solid feed, but this content is not insignificant. It typically represents 5 to 20% of the mass of dry coal. Some solid fuels such as pet coke have much lower ash contents. There are also solid fuels with higher ash contents.
TABLE 1Analysis of various coals—Coal ACoal BDry coalAshesWt. %10.314.8analysisVolatile matterWt. %37.624SulfurWt. %0.50.57Specific heatKcal/kg67106630UltimateCWt. %71.173.46analysisHWt. %4.773.87NWt. %1.411.65SWt. %0.50.57AshesWt. %10.314.76O (by difference)Wt. %11.925.69AshSiO2Wt. %6749.84compositionAl2O3Wt. %19.240.78Fe2O3Wt. %5.22.9CaOWt. %21.08MgOWt. %1.20.26TiO2Wt. %0.91.96K2OWt. %1.70.64Na2OWt. %1.70.06SO3Wt. %0.90.52P2O5Wt. %0.21.05
The ashes resulting from the combustion of coal are made up of residual fine particles. Their melting temperature varies according to their composition and it generally ranges between 1000° C. and 1500° C. However, at lower temperatures, for example between 800° C. and 1000° C., it is possible to observe a phenomenon of agglomeration of the ash particles that become sticky. They can therefore either agglomerate with one another, or they agglomerate with the particles of oxygen-carrying material. Considering the operating conditions in the chemical-looping combustion process, two types of ashes can be distinguished:                fly ashes: they correspond to the ashes that are carried to the fuel reactor by the combustion gases. Fly ashes generally represent 50% to 99% of the ashes formed (typically 70% to 90%). Their grain size is relatively fine with generally at least 25% fines with sizes below 10 microns and 90% fines with sizes below 100 microns. The Sauter mean diameter representative of the fly ash grain size generally ranges between 5 and 30 microns, and it is typically close to 10 microns. The grain density of these ashes generally ranges between 2000 and 3000 kg/m3, and it is generally close to 2500 kg/m3;        agglomerated ashes: they correspond to the ashes that agglomerate with one another or with the oxygen-carrying material and that are too heavy to be carried to the fuel reactor by the combustion gases. These ashes are sometimes referred to as agglomerated bed ash or bottom ash. In the present description, this type of agglomerated ashes is referred to as “agglomerated bottom ash”. The grain size of the agglomerated ashes is more delicate to estimate and depends on the conditions of implementation of the method. In general terms, the grain size of these ashes is estimated to be above 100 microns and their size can reach up to several millimeters.        
Agglomerated bottom ash thus forms dense objects larger than the oxygen carrier particles, with fluidization properties that can evolve and be different from those of the oxygen carrier. These objects may then be no longer fluidized at the same time as the transported bed and they accumulate in the bottom of the combustion reactor.
For the ashes that remain fluidizable under the operating conditions, specific devices allowing them to be eliminated have been proposed, as described in French patent FR-2,850,156, or in French patent applications FR-2,960,940 and FR-2,960,941. However, these devices work only for the fines produced by attrition of the oxygen-carrying material (through chemical or mechanical aging effect), the unburned solid fuel particles (“unburned particles”) and the fly ashes.
A specific device is necessary to discharge the agglomerated solids. A bottom valve open onto the fluidized bed and/or an endless screw directly supplied from the fluidized bed is generally used to discharge the agglomerated bottom ash from the lower part of the fluidized bed. Although these simple means provide a solution to the agglomerated bottom ash problem, they are by no means selective, and a larger proportion of oxygen carrier (predominant in the fluidized bed) than ashes proper will eventually be extracted.
An improved device is described in patent application FR-2,980,258. This document discloses a CLC plant wherein an agglomerated ash settling zone at the bottom of the combustion reactor comprises for example a cooled endless screw allowing a particle stream containing agglomerated ashes to be withdrawn. However, this device does not enable satisfactory extraction of the agglomerated bottom ash, which represents a small proportion of the particle stream withdrawn.
Non-selective or weakly selective extraction of agglomerated bottom ash can in the long term affect the process performances and increase the operating costs. Unintentionally extracted oxygen carrier particles may indeed represent a not insignificant part of the inventory that needs to be compensated by either supplying a new oxygen carrier or by recycling the oxygen carrier that has been separated from the ashes after cooling and screening, i.e. by external recycling.